Electrically tuned microwave bandpass filter using ferrites



Sept. 19, 1961 ELECTRICALLY TUNED MICROWAVE BANDPASS FILTER USING FERRITES Filed Jan. 22, 1959 F. REGGIA FIG? 4 FIGS FRANK REGGIA INVENTOR M5. 4 0- am Army Filed Jan. 22, 1959, Ser. No. 788,472 3 Claims. (Cl. 33373) (Granted under Title 35, US. Code (1952), see. 266) The invention described herein may be manufactured andused by or for the Government for governmental purposes without the payment to me of any royalty thereon.

This invention relates to microwave filters in general, and more particularly to a simple reciprocal microwave bandpass filter capable of being electrically tuned over a'wide frequency range.

Although electrically tuned microwave filters are known in the prior art, such filters are quite large and complex and it is usually necessary to make considerable changes in the waveguide structure in order to incorporate these filters in the waveguide. Also, the frequencyrange over which the filter can be electrically tuned has been very limited.

Accordingly, it is an object of this invention to provide an improved microwavebandpass filter which is simple, can easily be incorporated in a waveguide structure at any convenient location along the waveguideand, in addition, can be electrically tuned over a relatively large frequency range. Another object is to provide a microwave bandpass filter having the above features which also is reciprocal, that is, its operation as a filter is the same regardless of the direction of propagation through the filter.

A further object of this invention is to provide a micro wave bandpass filter in accordance with any or all of the above objects whose frequency can be made to either increase or decrease by the application of a suitable magnetic field.

Yet another object is to provide an electrically tuned microwave bandpass filter in accordance with any or all ofthe above objects which additionally provides a low insertion loss to microwave'frequencies within its passband and a relatively high rejection at frequencies outside of its passband.

United States Patent ice FIG. 1 is an end view of a rectangular waveguide in which an inverted cavity is formed by a ferrite rod having metallic reflecting plates at its ends, in accordance with the invention.

FIG. 2 is a sectional view taken along line 22 of FIG. 1 in which means for applying an axial magnetic field is additionally shown in schematic form.

FIGS. 3 and 4 are end views of a ferrite rod showing different sizes of metallic reflecting plates which may be used in accordance with the invention.

FIG. 5 is a side view of a form of ferrite rod with metallic reflecting elements which gives better reflection characteristics and a lower insertion loss because of the addition of tapered ferrite matching elements.

FIG. 6 is an end view of a rectangular waveguide incorporating a form of ferrite rod with metallic reflecting plates at its ends, which rod is particularly advantageous for use with transverse applied magnetic fields.

FIG. 7 is a sectional view taken along line 7-7 of FIG. 6.

FIG. 8 is a sectional view of an embodiment of the I invention in which two microwave filters are incorporated i In a typical embodiment'of the invention, the above 7 objects are accomplished by forming an inverted form of cavity in a rectangular waveguide through which the microwave energy it is desired to filter is propagated. The inverted cavity is formed by coaxially disposing in the rectangular wave guide, a ferrite rod having metallic reflecting plates at its ends. The dimensions of the ferrite rod and the size of the reflecting plates are chosen so that the portion of the rectangular waveguide between the metallic plates appears as a TE resonant cavity which propagates only a narrow band of frequencies in the vicinity of the resonant frequency of the cavity, all other frequencies being substantially rejected. The resonant frequency of the cavity can be'either increased or decreased by the application'of an axial or a transverse magnetic field, respectively, to the ferrite rod. Relatively small values of magnetic field of the order of about 100 oer steds for both axial and transverse applied directions have been found to produce a variation in the resonant frequency of the cavity of the order of 800 megacycles for resonant frequencies of the order of 10,000 megacycles.

The specific nature of the invention, as well as other objects, uses, and. advantages thereof, will clearly appear from the following description and from the accompanying drawing,inwhich:

in series in the rectangular waveguide.

In FIGS. 1 and 2, which illustrate a preferred embodiment of the invention, a ferrite rod 15 with metallic re fleeting plates '20 mounted at its ends is coaxially supported in a rectangular waveguide 12 by means of a suitable dielectric support 25. The reflecting plates 20 may be plated to the ends of the ferrite rod 15, cemented thereto, or mounted by any other suitable means. Means for applying an axial magnetic field to the ferrite rod 15 are provided by a coil 31 surrounding the rectangular waveguide :12 in any well known manner. The coil 31 is connected in series with a battery 30 and a switch 33, Closing the switch 33 causes a current to flow through the coils 12 setting up an axial magnetic field through the ferrite rod 15.

The dimensions of the ferrite rod 15 and the connecting plates 20 in FIGS. 1 and 2 are chosen so that the portion of the waveguide 12 between the metallic refleeting plates 20, as indicated by dashed lines 10, acts as a TE resonant cavity 29 at some predetermined micro- Wave frequency. Most of the energy in the cavity will be concentrated in the ferrite rod 15. Microwave energy applied to the waveguide 12 at the end 11 will thus only be propagated through the cavity 29 to the other end 13 of the rectangular waveguide 12 if the frequency of the microwave energy applied at the end 11 is within the narrow band of frequencies in the vicinity of the resonant frequency of the cavity 29. When the switch 33 is closed applying an axial magnetic field to the ferrite '15, a pro; portionate increase in the resonant frequency of the cavity 29 is produced. If a transverse magnetic field were applied to the ferrite rod 15 by well known means (such as shown in FIGS. 6 and 7) instead of the axial magnetic field, it is found that a proportionate decrease in the resonant frequency of the cavity 29 is produced. It can be seen, therefore, that changing the magnetic field applied to the ferrite rod 15 effectively results in varying the resonant frequency of the cavity 29, thereby permitting the passband of the filter to be placed at a desired location in the frequency spectrum. Also, since changes in the resonant frequency may be produced elec,- trically, rapid variations in the. resonant frequency may be produced. It will be understood that such operation is valuable for automatic frequency control systems, for rapid switching of microwave power in amplitude-modulated systems, and for tracking microwave oscillators used in frequency-modulated systems. j

Although the ferrite rod 15 is shown as having a rec,- tangular cross section and the metallic reflecting plates 20 are shown as completely covering the surfaces at trend. '0 the rennin 15, it

may be larger in area than the area of the surfaces at l .the ends of the ferrite rod 15, asshown in FIG. 3, or on the other hand, may be smaller in area than that of the ferrite rod 15, as shown in FIG. 4. Also, as illustrated in FIG. 5, a ferrite rod 95 having metallic reflecting plates 90 at its ends may be made to have better're fleeting characteristics and a somewhat lower insertion loss by mounting tapered ferrite elements '97 to the reflecting plates 9i), as shown. Also, the tapered elements 97 reduce the magnetic field necessary for a given frequency shift. It is to be understood that all possible versions of the ferrite within the scope of the invention will beconsidered included in the term ferrite rod. It is obvious that the dielectric supporting element 25, which may be Teflon orPolyfoam, for example, maybe provided in a'variety of other well known ways than that shown in FIGS. 1 and 2. Likewise, the means for applying a transverse magnetic field or an axial magnetic field, as the case may be, may readily be provided in a variety of ways by those skilledin the art.

' For the 3-centirneter band, the'following dimensions have been found to give good results in a specific construction of the preferred embodiment shown in FIGS. 1 and 2. In this specific construction, the ferrite rod has an axial length of .40 inch and a rectangular cross section of .30 by .20 inch. The metallic reflecting plates have a thickness of approximately .001 inch and are attached by plating on the ends of the ferrite rod 15. The

dielectric supporting element is of Teflon andis cemented to the ferrite rod 15. The unit comprising the ferrite rod 15 with its reflecting plates 20 and the supporting element 25 is slid into a rectangular waveguide '12 at any convenient location. For this specific construction, the resulting cavity 29 in the rectangular wave .g dc is resonant at about 9500' .mcgacycles with a bandwidth of about .100 megacycles. At resonance the voltage standing waveratio is less than 1.10 and the insertion loss is about one-half of a decibel. An axial magnetic field of about 100 .Oersteds applied to the ferrite rod 15 has been found to increase the resonant frequency of the cavity 29 of the order of .1009 megacycles, while a transverse applied magnetic field of 100 oersteds decreases the resonantfrequency by a like amount. With the magnetic frequency of 100 oer'steds applied, the insertion loss at the initial resonant frequency of 950% rnegacycles is greater than 20 decibels. It is tobe noted that no change the rectangular waveguide 12 is necessary except as 7 might be required for applying the magnetic field to the ferrite rod. I

.The characteristics of the resulting microwave bandpass filter are determined .by the construction of the ferrite rod 15 and the reflecting plates 20, or modifications thereof. The following discussion of the theory of operation of the invention and the effect of the sizes of the ferrite rod 15 and the reflecting plates 20, will enable one to determine the necessary dimensions to suit any particular application.

The resulting cavity 29 obtained by the embodiment of the invention illustrated inFIGS. 1 and 2, may be considered as an inverted form of cavity. The cavity is inverted in the sense that the metallic reflecting plates 2% serve as reflecting irises to reflect energy incident onthe cavity 29 which has a frequency diflerent from the reso- 3m frequency thereof. Microwave energy having a frequency equal to the resonantfrequency is coupled to the cavity 29 around the reflecting metallic plates. 2%. The conventional form of cavity, on the other hand, is

' substantially closed and has openings therein'which serve :as Input output irises to couple the energy into and out of the cavity. Reflection of microwave energy out-' the case of the inverted cavity 29 in FIGS. 1 and 2.

Resonant frequency energy is coupled to the conventional closed cavity by means of its in'ses while in the inverted cavity of FIGS. 1 and '2, the coupling is around the metal irises 20. It can be seen, therefore, that the term inverted cavity is quite appropriate for the type of cavity formed in accordance with this invention, as exemplified in FIGS. 1 and 2. w

The dimensions of the ferrite rod 15 and the reflecting plates 26 at its ends are chosen so that the inverted cavity thereby formed is resonant at the TE mode at some desired frequency. The effective axial lengthof the invcrted cavity is thus equal to one-half wavelength at the cavitys resonant frequency. .Since the reflecting plates 20 are the means by which frequencies outside the resonant frequency of the invelted cavity are reflected, the

size of these platesdetermine the Q and thus the bandwidth of the cavity. The greater the area of the reflect ing plates 20, the higher the Q and the narrower, the bandpass, and vice versa; The volume of the ferrite in relation to-the total volume of the cavity has been found to determine the amount of variation in the resonant frequencyof the cavity caused bychanging the magnetic field applied to the ferrite rod 15. The greater the volumegof the ferrite rod 15 compared to the volume of the cavity, the larger, the resulting change produced by an applied magnetic field. The main restriction-on the factor determining the mode of the cavity, only a single polarization can be propagated therethrough. Another factor to be considered in dimensioning the ferrite rod is that the greater the volume of the ferrite rod, the greater the insertion loss that occurs. In summary, it can "be stated, therefore, that the area of the reflecting plates determines the bandwidth of'the cavity, while the volume of ferrite relative to the volume of the cavity determines how much of an effect a change in magnetic field will produce in the resonant frequency of the cavity. from the above considerations, thoseskilled in the 'art ill readily be able to design "an electrically tunedv microwave bandpass filter in accordance with the invention to have a desired resonant frequency, bandpass, and weifective change in resonant frequency with applied magnetic field.

are mounted at the end surfaces of the rod and coverv only a portion of the surfaces thereof. A soft. iron 16. core has a coil 131 around its leg which is in. series with a battery and a switch 133. Closing the switch 133 applies a current to the coil, causing a transverse magnetic field to be applied to the ferrite rod'105. The advantage of the arrangement shown in FIGS. 6 and 7 is that the eifective air gap between the ferrite rod 105 and the ends 126 and 12.7 of. the C-core 125 is very greatly reduced. This makes it possible to obtaingreater, shifts 1n the resonant frequency of the inverted c v y with smaller applied magnetic fields.

FIG. 8 shows an embodiment of the inventionin which two filters are connected in series to increase the insertion "loss at frequencies out of the desired bandpass. In this arrangement, two ferrite rods 55 and 75 .are coaxially supported in the rectangular waveguide 12 by dielectric supporting elements 73 and 7 1, respectively. A metallic reflecting plate 62 is mounted between and in contact in accordance with the considerations previously given.

It is apparent that either an axial or a transverse magnetic field may be applied to ferrite rods 55 and 75.

It will be apparent that the embodiments shown are I only exemplary and that various modifications can be made in construction and arrangement within the scope of the invention as defined in the appended claims.

1 claim as my invention:

1. A continuous rectangular waveguide having an input end and an output end, means for coupling input energy at a frequency within a particular band of frequencies to the input end of said waveguide, and filter means inserted in the center of a section of said waveguide -for passing only energy within said band of frequencies to the output end of said waveguide, said filter means consisting of: a ferrite rod mounted in said waveguide section, metal reflecting plates mounted on said rod and spaced from each other in the direction of propagation of electromagnetic energy in said waveguide, and electrical means for applying a variable magnetic field to said rod.

2. The invention in accordance with claim 1 wherein said ferrite rod has a rectangular cross section, wherein two inverted cavities in series, each one being designed said reflecting plates completely cover the rectangular surfaces at the ends of said rod, and wherein said electrical means is adapted to apply an axial magnetic field to said rod.

3. The invention in accordance with claim 1 wherein said ferrite rod extends across the short side of said rectangular waveguide, and wherein said electrical means is adapted to apply a transverse magnetic field to said rod.

References Cited in the file of this patent UNITED STATES PATENTS 2,704,830 Rosencrans Mar. 22, 1955 2,752,495 Kroger June 26, 1956 2,890,422 Schlicke June 9, 1959 2,909,738 Davis Oct. 20,

FOREIGN PATENTS 1,079,880 France June 29, 1955 (First addition to No. 64,770) OTHER REFERENCES 

